A magnetic tunnel junction (MTJ) device is basically a variable resistor including two ferromagnetic layers and a tunnel barrier sandwiched between them. The relative magnetization orientation of the two ferromagnetic layers causes electrons to pass through the tunnel barrier and make the spin-polarized electrons have different tunnel probabilities, thereby changing the resistance.
The tunnel barrier is generally made of an insulating material and must be very thin and have uniform thickness and structure. Nonuniformity of the tunnel barrier in chemical composition and thickness greatly degrades the device performance.
From the advent of MTJ devices, the development of MTJ devices having high TMR ratios at room temperature has been industrially a host issue in order to implement spintronic applications such as a nonvolatile magnetic random access memory and a hard disk read head having a surface density exceeding 100 Gbits/in2 (Moodera et al., Phys. Rev. Lett., 74 (1995), p 3273).
During the initial period, high TMR ratios are achieved by a ferromagnetic electrode layer with a high spin polarization and an amorphous AlOx tunnel barrier. The highest TMR ratio of this arrangement at room temperature was about 70%. Thereafter, a spin filtering effect using a single-crystal MgO tunnel barrier having an NaCl structure was proposed by theoretical calculations (Butler et al., Phys. Rev., B63 (2001), p 054416). It was expected that a TMR ratio as high as 6,000% at room temperature was achieved.
The crystal structure of single-crystal MgO has a four-time symmetry. The state of electrons having a tunneling probability enough to pass through MgO is the Δ1 state having a four-time symmetry. For this reason, in a MTJ having a single-crystal Fe/MgO/Fe structure, conduction by the Δ1 band is dominant. However, since the Δ1 band in Fe is 100% spin-polarized at the Fermi level, a sufficient tunneling probability cannot be obtained in the antiparallel magnetization of the MTJ. That is, MgO has an effect of spin filtering depending on the magnetization state.
This allows coherent tunneling and can provide a higher TMR ratio. To achieve this higher TMR ratio, an experiment was conducted to grow single-crystal Fe/MgO/Fe by molecular beam epitaxy. This experiment exhibited a TMR ratio of 180% at room temperature (Yuasa et al., Nature materials, 3 (2004)).
A TMR ratio of 220% at room temperature was reported for a combination of a polycrystalline CoFe ferromagnetic electrode and an MgO tunnel barrier (Parkin et al., Nat. Mater., 3 (2004), p 862) A higher TMR ratio was reported in a MTJ having a combination of amorphous CoFeB and an MgO tunnel barrier on a silicon substrate having a thermal oxide in accordance with a magnetron sputtering method serving as a practical deposition method (Djayaprawira et al., Appl. Phys. Lett., 86 (2005), p 092502).
Many efforts have been made to form a MTJ tunnel barrier which is very thin and has uniform thickness and composition. An important point in the manufacture of an oxide tunnel barrier is to prevent surface oxidation of a ferromagnetic electrode layer serving as an underlayer of the tunnel barrier and prevent nonuniformity of an oxygen profile in the oxide tunnel barrier.
Tunnel barrier deposition methods are generally classified into a method of directly depositing an oxide and a method of depositing a metal and then oxidizing the metal. Examples of the direct deposition method are RF-sputtering using an oxide target and reactive sputtering for sputtering a metal target in an oxygen atmosphere. Examples of the method of depositing a metal and then oxidizing the metal are native oxidation, plasma oxidation, radical oxidation, and ozone oxidation.